1. Field of the Invention
This invention relates to an apparatus and a method for multiplexing signals of different frequency bands, and, in particular, to a low cost signal multiplexer and method that exhibits substantially no reflected energy to the input ports.
2. Description of the Prior Art
Prior art multiplexers include a branch combiner, a star combiner and a manifold combiner. These signal combiners have a serious disadvantage in that they do not present a constant impedance to the inputs that receive the signals. That is, out of band energy is reflected back to the inputs.
Another prior art multiplexer presents a constant impedance to the input. An example of this style of multiplexer is shown in U.S. Pat. No. 4,029,902. As shown in this patent, this style of multiplexer typically has a pair of matched filters, two hybrids and a load for each signal channel. The filtered output of each channel is fed as an input to the next channel and so on. Although reflected energy to the channel inputs is nil, the cost is high because 2N hybrids are required, where N is the number of signals or channels to be multiplexed. Moreover, the bandwidth is limited to the short circuit VSWR (voltage standing wave ratio) of the output hybrid. The short circuit VSWR is dependent on the amplitude balance of the output hybrid of each channel. In coax hybrids, the amplitude balance is controlled in the design of the hybrid by the use of multiple quarter wave coupled sections, which are power limited. In wave guide hybrids, which can take more power, the amplitude balance is limited by the inability to produce multiple quarter wave coupled sections. Consequently, when the bandwidth causes the amplitude ripple to exceed 0.085dB, the short circuit VSWR exceeds 1.04:1. The sum of the VSWR of the individual channels quickly accumulates until the system exceeds a VSWR of 1.10:1that is required by the transmitters.
When installed, a television broadcast system needs to be capable of testing the transmitter of any channel without disturbing broadcast activity of any other channel. The tests are conducted, for example, for optimizing transmitter performance as well as for compliance with the requirements of governmental regulatory agencies, such as the Federal Communications Commission.
There is a need for a low cost constant input impedance multiplexer as well as a need for a constant impedance multiplexer that is not bandwidth limited by the quality or power capacity of components, such as hybrid couplers.
There is also a need for a multiplexer with a test capability for any selected channel without disturbing broadcast activity on other channels.
A multiplexer according to the invention uses only N+1 hybrid couplers to multiplex N signals, while limiting reflected energy to substantially zero at the inputs that receive the signals being multiplexed. The multiplexer includes N hybrid couplers for converting each of the N signals into a pair of quadrature phase signals. There is a pair of matched band pass filters for each signal or channel that filter the quadrature phase signals. The pairs of filtered quadrature phase signals for all the channels are combined in a signal combiner to produce a pair of N channel multiplexed quadrature phase signals. A single output hybrid coupler converts the pair of N frequency band multiplexed quadrature phase signals into a single multiplexed signal.
Each of the N hybrid couplers has a first port that receives one of the N signals. The remaining ports of the N hybrid couplers are connected in a manner to prevent a reflection of energy to the corresponding first port. To this end, each of the hybrid couplers has a second port connected with a load, and third and fourth ports connected with separate ones of the associated pair of matched band pass filters, whereby out of band energy reflected from the band pass filters is steered to the load.
The signal combiner includes a first signal combiner for combining all of the filtered quadrature phase signals of one of the phases and a second signal combiner for combining all of the filtered quadrature phase signals of the other phase. In one embodiment, the first and second signal combiners are first and second manifolds, respectively.
The method according to the invention, converts each of the N signals into a pair of quadrature phase signals. The quadrature phase signals are then filtered. The N pairs of filtered quadrature phase signals are combined to produce a pair of N frequency band multiplexed quadrature phase signals. The pair of N frequency band multiplexed quadrature phase signals are then converted into a single multiplexed signal.
In an alternate embodiment of the invention, the signal multiplexer includes a selector mechanism that selects one of the N signals. The N signals are filtered and combined to produce a pair of multiplexed quadrature phase signals as described above for the first embodiment. However, the output hybrid coupler converts the pair of multiplexed quadrature phase signals into a first multiplexed signal that contains the frequency band of the selected signal and a second multiplexed signal that contains the frequency bands of the non-selected signals. This allows the transmitter of the selected signal to be tested with measurements of the first multiplexed signal.